Acoustic emission linear pulse holography

Abstract

Defects in a structure are imaged as they propagate, using their emitted acoustic energy as a monitored source. Short bursts of acoustic energy propagate through the structure to a discrete element receiver array. A reference timing transducer located between the array and the inspection zone initiates a series of time-of-flight measurements. A resulting series of time-of-flight measurements are then treated as aperture data and are transferred to a computer for reconstruction of a synthetic linear holographic image. The images can be displayed and stored as a record of defect growth.

We claim: 
     
       1. A method for monitoring growing defects in an inspection zone within a structure, comprising the following steps: mounting a plurality of discrete sensors, including a reference sensor, at fixed locations in a preselected array along a surface of the structure adjacent to the inspection zone;   detecting the acoustic waves emitted from a growing defect in the inspection zone as output pulses generated at each sensor;   determining the phase components of the output pulse generated at each sensor at a selected time as a function of the pulse transmission time between the sensor and the reference sensor and as a function of a synthetic frequency selected from the pulse bandwidth; and   combining the resulting cumulative phase information into a synthetic hologram of the defect.   
     
     
       2. The method of claim 1, wherein the step of determining the phase components of the output pulse is repeated at selected intervals over a period of time. 
     
     
       3. The method of claim 1, further comprising the following step: reconstructing the synthetic hologram into an image of the defect.   
     
     
       4. The method of claim 1, further comprising the following step: reconstructing the hologram into sequential images of the defect at preselected depth intervals within the structure; and   determining the depth of the defect relative to the surface of the structure on which the sensors are mounted by identifying the depth at which the sequential image reconstruction achieves optimum focus.   
     
     
       5. An apparatus for monitoring growing defects in an inspection zone within a structure, comprising: a plurality of discrete sensors, including a reference sensor, mounted at fixed locations in a preselected array along a surface of the structure adjacent to the inspection zone, each sensor having the capability of detecting acoustic waves emitted from a growing defect within the structure as an output pulse;   means for determining the phase components of the output pulses generated at each sensor at a selected time as a function of the pulse transmission time between the sensor and the reference sensor and as a function of a synthetic frequency selected from the pulse bandwidth;   and means for combining the resulting cumulative phase information into a synthetic hologram of the defect.